Lime control method for highly alkaline flotation pulps

ABSTRACT

Alkalinity of the flotation pulp in a high alkalinity flotation circuit is controlled automatically by continuously measuring electrical conductivity of such pulp and controlling the addition of lime to the circuit in accordance with changes in the conductivity resulting from deviations of alkalinity of the pulp from a pre-established norm.

United States Patent Moon et al.

[451 Oct. 29, 1974 Primary ExaminerHenry T. Klinksiek Attorney, Agent, or FirmMallinckrodt &

[75] Inventors: Anthony G. Moon; Roger L.

Vaughn, both of Silver City, N. Manmckrodt Mex.

[73] Assignee: Kennecott Copper Corporation,

New York, NY. [57] ABSTRACT [22] Filed: Dec. 13, 1972 Alkalinity of the flotation pulp in a high alkalinity flotation circuit is controlled automatically by continu- [21] Appl' 314540 ously measuring electrical conductivity of such pulp and controlling the addition of lime to the circuit in [52] U.S. CI. 137/5, 137/93 accordance with changes in the conductivity resulting [51] Int. Cl. G05d 11/02 from deviations of alkalinity of the pulp from a pre- [58] Field of Search l37/5, 93; 75/2 established norm.

[ References Cited 3 Claims, 1 Drawing Figure UNITED STATES PATENTS 1,450,023 3/1923 Edelman 137/93 CONDUCTIVITY MEASUREMENT INSTRUMENTATION coiiii iiioR AND CONTROLLER COMPRESSED AIR /3 SUPPLY OF MILK OF LIME o FLOTATION srsrEM (eq. BALL MILL) Pmmtuumsmu E 3344.303

CONDUCTINSITYY MEASUREMENT INSTRUMENTATION SIGNAL COMPARATOR AND CONTROLLER 3 SUPPLY OF MILK OF LIME J COMPRESSED AIR- S FLOTATION SYSTEM TO BALL MILL) LIME CONTROL METHOD FOR HIGHLY ALKALINE FLOTATION PULPS BACKGROUND OF THE INVENTION 1. Field The invention is in the field of alkalinity control of a flotation pulp by the taking of measurements indicative of pulp alkalinity and by the governing of the quantity of lime additions to the pulp in accordance with such measurements in order to maintain the pulp at a predetermined alkalinity during operation of a flotation circuit.

2. State of the Art The proper and effective control of conditions in a flotation circuit, wherein certain mineral values are being separated from other mineral values and from associated gangue materials, is extremely important in the industrial milling of ores for recovery of desired values therefrom. Since the percentage of desired values in an ore material is often very low as compared to the materials that must be rejected in the concentrating of the desired values by flotation, considerable amounts of money and sometimes the very existence of a mining venture are at stake in the effectiveness of operating procedures applied during the milling of the ore in order to recover, in a mineral concentrate, the maximum amount of the desired values present in the ore.

flotation process, and led to the use of various selective agents, such as sulfites, cyanides, and sulfides, as reagents designed to permit the flotation of certain sulfide minerals while inhibiting the flotation of others. Thus so-called selective flotation was introduced about the year 1921. Alkalinity of flotation pulps has been controlled since that time primarily by the addition of more or less lime to the pulp, and the degree of alkalinity has been maintained as constant as possible at a value deemed desirable for a particular operation by testing the pulp in a flotation circuit for alkalinity from time to time during the course of the flotation operation.

It has been customary to measure the degree of alkalinity of a pulp in a flotation circuit by titration techniques performed manually at frequent intervals during the course of the flotation operation and to add more or less lime in accordance with the degree of alkalinity so determined. Automatic titraters have been developed for instances in which automatic control of lime addition has been desired. Direct measurement of the alkalinity is also possible by the use of so-called pH electrodes", and alkalinity of flotation pulps has been monitored continuously by the use of such electrodes. However, these have-proven unsatisfactory for pulps of higher alkalinity, since very small changes in pH require relatively great quantities of lime to be added to the pulp.

Attention is called to a U.S. Pat. No. 1,450,023, issued to Philip E. Edelman on March 27, 1923, on an application filed of December of l9l9. This patent advocates the use of changes in the electrical conductivity of a material for the automatic control of reactions, mixtures, and the like in a great variety of industrial processes, and discloses a system whereby electrical conductivity measurements of a material are made continuously and are utilized to control various industrial operations, including the addition of reactants to the material concerned, by comparing such measurements with a preset standard and providing for the addition of only so much of the added material as will maintain the electrical conductivity of the material concerned at substantially the preset value. The control of acidity or alkalinity of solutions is referred to in general in this patent, as is the control of oil, acid, and water supply in the overflow froth of a flotation process and the control of density of feeds in flotation work. Milk of lime is also referred to as a reagent that may be controlled in its addition to water for the purification thereof.

It would be supposed that the issuance of the Edelman disclosure so close on the heels of the commencement of use by the flotation art of alkaline circuits would have immediately led to the use of electrical conductivity measurements for the automatic control of lime additions to flotation pulps. But it did not, and the flotation art has not found it obvious to apply conductivity measurements to the control of such lime additions. One of the reasons for this is undoubtedly the fact that it has been supposed that the inevitable variations in ore characteristics from time to time in a flotation circuit would make it impossible from a practical standpoint to utilize present values of electrical conductivity as a control criterion.

SUMMARY OF THE INVENTION In accordance with the present invention, we have found that ore variability does not have any appreciable effect on conductivity of a flotation pulp or slurry containing lime and, just as important, that electrical conductivity is a linear function of lime concentration in pulps of higher alkalinity. This is particularly true with flotation pulps having a pH of about 11 and above, but also holds true for pulps having a pH of about 9 and above. Between a pH of 8 and 9 electrical conductivity measurements as measures of alkalinity of a flotation pulp become unreliable.

In carrying out the invention, measurements of the electrical conductivity of flotation pulp in an alkaline flotation circuit are tkaen continuously from within the body of pulp in a flotation circuit by means of any suitable conductivity measurement instrumentation, and are utilized, by means of suitable electrical comparator and control instrumentation, to operate valve means controlling lime addition to the circuit.

Degree of alkalinity of an ore pulp is one of the operating parameters normally pre-established for any given flotation circuit to achieve effective metallurgical results with respect to the particular ore concerned. It is highly desirable that such pre-established degree of alkalinity be maintained throughout operation of the circuit. In carrying out the invention, a flotation pulp of a sulfide ore, such as an ore containing copper sulfide minerals as the desired values, is brought to the desired degree of alkalinity by appropriate additions of lime and is then tested for electrical conductivity in order to determine a conductivity norm against which measured values of electrical conductivity can be equated for control purposes. For control purposes, the electrical comparator and controller instrumentation is DRAWING The best mode presently contemplated of carrying out the invention is illustrated in the accompanying drawing, in which the single FIGURE is a schematic showing of a particular conductivity measurement and control system applied to a flotation circuit for controlling milk of lime additions to the circuit.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT In the illustrated system, milk of lime is supplied to an appropriate part of a flotation system, e.g., a ball mill, from a suitable source of same through piping l controlled by a conventional diaphragm valve, as indicated. As shown, such valve is operated by pressure air supplied from any suitable source of compressed air. through piping ll under the control of a standard solenoid valve 12, which is operated by electrical power supplied through electrical lines 13 from an ordinary I volt power supply (not shown) under the control of an output relay (not shown) constituting part of the signal comparator and controller, which is indicated as such. It is preferred to use instrumentation of the three mode control type manufactured by Minneapolis- Honeywell under the trade designation Electr-o- Pulse" controller, having built-in pulsing circuits. Such instrumentation automatically produces a pulse output proportional to the control requirements obtained through comparison of the input signals with a preset standard indicative of the alkalinity to be maintained in the flotation circuit, which circuit includes a distributor for feeding flotation pulp or slurry to several rows of flotation cells (not shown).

The solenoid valve 12 operates on a fixed cycle of opening and closing, typically a cycle of 30 seconds. The duration of the valve opening portion of the cycle relative to the valve closing portion is, however, dependent upon the pulse output of the signal comparator and controller, and, at any given time, this is dependent upon the comparison made by such instrumentation between the measured conductivity value and the preset conductivity value. The quantities of milk of lime supplied through the diaphragm valve to the flotation circuit from time to time are dependent upon the durations of the valve opening portions of the respective valve opening and closing cycles.

The conductivity measurement instrumentation is preferably an electrodeless conductivity meter of the type manufactured by Beckman Instrument Co. and marketed under the designation RS 4 Conductivity Indicator/Transmitter" adapted to supply signals of from 0 to 10 millivolts to the signal comparator and controller through the transmission lines 15.

A dip type conductivity sensor with built-in automatic temperature compensation, see 16, is normally supplied with the Beckman conductivity measuring instrument and has worked very satisfactorily when placed in a body of the ore pulp at least twelve inches from any structure.

We have found that electrical conductivity of the lime-containing ore pulp bears a linear relationship to the alkalinity thereof at pH values of about nine and above but is independent of ore characteristics. However, a sensor of the type indicated cannot discriminate against magnetite. Accordingly, when magnetite is present in the ore in this embodiment of the apparatus of the invention, it may be advisable to monitor the flotation pulp periodically (e.g. every 2 hours) by titration to determine whether the preset standard conductivity value in the signal comparator and controller should be changed. This can be avoided if a magnetite measuring device is included in the control circuitry and arranged to automatically change the preset standard from time to time.

The location of the conductivity sensor, e.g. the sensor 16, in the pulp or slurry going to the flotation cells of a flotation circuit whose alkalinity is being controlled in accordance with the invention is important for accuracy of the conductivity measurements made. It has been found that the sensor can be suspended most anywhere in the system for feeding flotation pulp or slurry to the flotation cells, so long as it is free and clear of structural surfaces, preferably at least 12 inches therefrom. An ideal location is in the distributor, as at I4, commonly utilized in a flotation circuit to direct the slurry feed to different lines of flotation cells. Care should be taken to place the sensor in a location in which it will be submerged in the feed pulp or slurry, but not where solids would tend to build-up therein. Some testing may be required to find the best location in any given flotation application.

In accordance with the invention, it is possible to maintain the alkalinity of the flotation pulp or slurry within a range of plus or minus one alkalinity unit for approximately percent of the time under normal operating conditions. When major processing changes occur, such as a ball mill shut down or a choked ore bin feeder, alkalinity variations plus or minus two alkalinity units may result for a short time, e.g. 30 minutes. The alkalinity of a pulp or slurry is defined as the quantity of N/I0 sulfuric acid, in milliliters, required to neutralize milliliters of water decanted from such pulp or slurry. Equated to normal pH usage, the alkalinity of the flotation pulp or slurry can be maintained within a range of plus or minus 0.05 of a pH unit under normal operating conditions.

As an example of the effectiveness of the invention, readings made from slurry samples taken periodically throughout a typical operating day from each of two parallel rougher flotation stages and from regrind for two cleaner stages of a typical flotation operation are shown in the following table:

Time Stages Circuits 7:30 am. 9:30 I 1:30 1:30 3:30 5:30 7:30 9:30 1 1:30 I230 -Continued TYPICAL ALKALINITY READINGS. AUGUST 30. W72

Parallel Flotation Cleaner Rcgrind Time Stages Circuits Average 9.1 8.2 20.7 V9.8

sensing the electrical conductivity of the flotation pulp in said circuit; continuously comparing the values of electrical conductivity so obtained with a preset value of electrical conductivity corresponding to said predetermined degree of alkalinity; and controlling lime additions to said flotation circuit in accordance with deviations of the sensed values of electrical conductivity from said preset value.

2. A method in accordance with claim 1, wherein the predetermined degree of alkalinity to be maintained is at or above a pH of about 1 1.5.

3. A method in accordance with claim 1, wherein the flotation pulp contains a significant quantity of magnetite; the pulp is monitored for the effect of such magnetite on the conductivity measurements; and the preset value of electrical conductivity is changed from time to time in accordance with quantity of magnetite in the 

1. A method of controlling lime additions to an alkaline flotation circuit operating on a sulfide mineral ore in order to substantially maintain a predetermined degree of alkalinity in the flotation pulp of such circuit at or above a pH of about 9, comprising continuously sensing the electrical conductivity of the flotation pulp in said circuit; continuously comparing the values of electrical conductivity so obtained with a preset value of electrical conductivity corresponding to said predetermined degree of alkalinity; and controlling lime additions to said flotation circuit in accordance with deviations of the sensed values of electrical conductivity from said preset value.
 2. A method in accordance with claim 1, wherein the predetermined degree of alkalinity to be maintained is at or above a pH of about 11.5.
 3. A method in accordance with claim 1, wherein the flotation pulp contains a significant quantity of magnetite; the pulp is monitored for the effect of such magnetite on the conductivity measurements; and the preset value of electrical conductivity is changed from time to time in accordance with quantity of magnetite in the ore. 